Nitrogen oxides (NOx) in the flue gases of combustion processes constitute a considerable environmental hazard. These gases are generated, for example, in the motors of vehicles, such as diesel cars. The amount of nitrogen oxides contained in the exhaust gases in the flue gases can be decreased by reducing them into nitrogen gases (N2). Conventionally, in what is called a selective catalytic reduction (SCR), a urea solution with a concentration of 32.5% has been used to decrease the amount of NOx gases, the solution being fed into an exhaust manifold before an SCR catalytic converter. The ammonia that is released from the solution reduces the NOx gases contained in the exhaust gases and, as a result of the reduction reaction, nitrogen (N2) and water exit the catalytic converter.
The previous patent applications of the applicant, FI20030168 and FI20041057 disclose alternatives for the urea solution, using, instead of the urea solution, an aqueous solution containing urea and ammonium formate. Such a solution has the advantage, among others, that its frost resistance is better than that of the urea solution. Using the solution, frost resistances of as low as −30° C. are achieved, whereas those of the conventional urea solution are only −11° C. at the lowest. In certain applications, the solution formed by the mixture of urea and ammonium formate can be used alternatively instead of or in addition to the standard urea solution. In that case, by changing the mixture ratio by increasing the portion of ammonium formate, for example, the efficiency of the solution can be modified for various applications. The solution compound can contain an essentially larger amount of usable ammonia than the urea solution alone, whereby the catalyst solution in question can be used for driving a longer distance by one refueling. In heavy traffic applications, a considerably longer distance can thus be driven using the same tank size without refueling, compared to the urea solution, or in passenger car applications, the refueling can be optimized to be carried out in connection with other maintenance procedures, whereby the maintenance intervals can be as long as 20000-30000 km. Such a compound can also be used to exploit the stability of ammonium formate that is better than that of urea in storage, and its preservation properties in preventing the contaminations of the solution compound. Furthermore, ammonium formate decomposes completely even at low temperatures, thus enabling a better conversion, which provides the benefit that the deposits accumulating in the exhaust manifold decrease compared to, for example, the use of pure urea solutions. The cubic expansion occurring in connection with freezing is also lesser for the solution compound compared to water or urea solutions.
In the selective catalytic reduction, the reducing agent that is fed into the exhaust manifold should be extremely pure to prevent the deactivation of the SCR catalyst. The catalytic converter must withstand automobile use as long as possible, so that the expensive catalytic converter does not have to be replaced during the service life of the car. Combustion of diesel and the use of lubricating oils also bring deactivating components to the catalyst; therefore, it is appropriate to try to minimize the amounts of substances that come along with the reducing agent and deactivate the catalyst. Accordingly, the reducing agent composition that is used should contain as few components as possible, which deactivate the catalyst or bring to the exhaust gases other undesired products, which are harmful to humans or environment. In practice, the disintegration products of the reducing agent are allowed to include water and carbon dioxide only, and, of course, ammonia, which, when reducing, is converted into nitrogen gas.
For the urea solution, there is a German DIN standard, which defines the degree of purity required. An ISO standard for the urea solution is also in preparation, its purity requirements being even stricter than those of the DIN standard. On completion of the ISO standard, the requirement of purity will most likely be in accordance with Table 1.
TABLE 1LimitPropertyUnitMinMaxUrea contenta,e% (m/m)d31.833.2Density 20° C.b,ekg/cm310871093Refractive index 20° C.c,e—1.38141.3843Alkalinity as NH3e% (m/m)d—0.2Biurete% (m/m)d—0.3Aldehydesemg/kg—5Insolublesemg/kg—20Phosphate (PO4)emg/kg—0.5Calciummg/kg—0.5Ironmg/kg—0.5Coppermg/kg—0.2Zincmg/kg—0.2Chromemg/kg—0.2Nickelmg/kg—0.2Aluminumemg/kg—0.5Magnesiummg/kg—0.5Sodiummg/kg—0.5Potassiummg/kg—0.5Identifiabilitye—Identical comparedto the referencewhereinaTarget value is 32.5% (m/m)bTarget value is 1089.5 kg/m3cTarget value is 1.3829dTerm “% (m/m)” is used to describe the mass ratio of the materialeRequirement according to the future ISO standard.
These very strict purity requirements limit the purity of the raw materials used and set strict requirements for the material. The ISO standard will include recommendations for the structural materials used.
Correspondingly, when replacing the conventional urea solution with the reducing agent composition containing urea and formate, it should meet the same purity criteria, which sets limits on the raw materials and the equipment that are used in the preparation of the solution composition.
Solid urea, which is mainly used in fertilizing, is conventionally quite impure, and because of its hygroscopicity, the product must often be coated to prevent caking. The most common coating agents of urea contain formaldehyde, urea-formaldehyde, and surfactants, such as alkyl acryl sulfonates, their amounts being strictly defined by the standards mentioned above. In practice, the purity requirements described above prevent the use of coated urea as such in the preparation of the solution-like reducing agent.
When preparing the reducing agent composition containing urea and ammonium formate, the urea starting material that is used must thus comprise pure uncoated urea, such as technical grade urea or a strong urea solution directly from a urea plant to achieve the required purity. Naturally, this keeps the material costs high. In practice, the use of normal bulk raw material exceeds the allowable purity criteria, whereby the only alternative is to use raw material that is delivered in flexible intermediate bulk containers or freight containers, or to introduce raw material in liquid form directly from the urea plant.
Patent specification GB1111936 describes tests for decreasing the freezing point by using mixtures of urea and ammonium formate. The essentially solid starting materials are simply mixed together and the solution is dissolved in water.
Several patent specifications, such as U.S. Pat. No. 6,387,336 and WO9217402, describe separate solutions of urea and ammonium formate and combined solutions, which are prepared by first dissolving the solid starting material, urea or ammonium formate, in water, and then combining the aqueous solutions thus obtained.
Patent specification U.S. Pat. No. 3,122,584 discloses the preparation of ammonium formate by hydrolyzing from methyl formate in the presence of an acid catalyst and by adding ammonia to the reaction mixture to provide a mixture containing ammonium formate. The ammonium formate formed in the method described herein is separated out of the reaction mixture, typically, by centrifuging the evaporated reaction mixture and recovering the ammonium formate crystals by filtering for subsequent use.
As is well known, the aqueous solution formed by ammonium formate and urea has been prepared from solid starting materials, solid urea and solid ammonium formate by dissolving them in water.
The preparation of solid or concentrated ammonium formate consumes energy because of the great need of evaporation and, furthermore, concentrated ammonium formate of more than 50% is difficult to handle because of its tendency to crystallize.
Correspondingly, the direct mixing of ammonia and formic acid with the aqueous solution of urea easily results in an increase in temperature, whereby the urea begins to decompose. In that case, the control of the exact composition of the end product becomes difficult and the end result might be a non-homogeneous product.
The purpose of the present invention is to disclose a method for preparing a reducing agent composition that is used in the catalytic reduction of nitrogen oxides, avoiding the problems described above. In particular, the purpose is to disclose a method of preparing the reducing agent compositions described in patent applications FI20030168 and FI20041057.
Another purpose of the invention is to disclose a method, which provides a reducing agent composition that meets the purity requirements according to those described above.
In the literature, no manufacturing method for such a reducing agent composition has been disclosed. Neither are there available any information about the solubility of a ternary system formed by urea, ammonium formate, and water.